Carburetor metering systems

ABSTRACT

A carburettor metering system comprises a fuel evaporator 113 consisting of porous parallel plates 114 with their lower portions immersed in fuel 121, fuel metering means 122 for supplying fuel to the evaporator 113, and a laminar flow air restrictor 128 comprising a series of parallel plates 127 separated by spacers and defining narrow gaps between the plates 127. Such an arrangement enables a substantially constant air/fuel mixture strength to be obtained over a wide range of air flow rates in single cylinder engines. Furthermore the supply of mixture to the engine by way of an exit tube 126 may be controlled by a valve member 132 coupled to the engine governor so that the rate of flow of mixture varies in dependence on the load, and additionally so as to change the system between two modes of carburettor operation, namely lean operation, which is provided up to about three quarters load, and rich operation in which additional fuel is supplied to the evaporator 113.

This invention relates to carburettor metering systems and is concernedmore particularly, but not exclusively, with carburettor meteringsystems for supplying air/fuel mixture to small gasoline engines, whoseexhaust emissions are subject to legislative control.

Small engines are typically single cylinder four-stroke engines of lowcost, and are used in applications such as lawnmowers and outboardmotors. Such single cylinder engines draw in air/fuel mixtureintermittently, and this causes problems in fuel metering which are notpresent in multi-cylinder engines such as are typically used in theautomotive field. Furthermore, small engines, unlike automotive engines,operate either at a governor controlled speed or with a fixedrelationship between speed and load, and usually have fixed ignitiontiming. Existing carburettor metering systems for such small engineshave a tendency to produce inhomogeneous air/fuel mixtures containingunevaporated fuel droplets which tend to increase the quantity ofhydrocarbons in the exhaust emissions. It is also known that ahomogeneous mixture allows operation with a weak air/fuel mixture, atpart load, which has the advantage of reducing emissions of oxides ofnitrogen and of carbon monoxide.

Generally carburettor metering systems produce a pressure differencerelated to air flow, and use this pressure difference to propel fuelfrom a constant pressure container into the air flow, usually as a moreor less atomised spray. In order to produce an air/fuel mixture ofhighly uniform consistency, with the above mentioned benefits in termsof exhaust emissions, it is known to supply the fuel to a fabric wickthrough which the warmed air is passed to produce a substantially dryfuel vapour. In such systems, however, it is preferable that the powercontrol throttle is placed downwind of the wick so that there are nosignificant pressure changes at the wick when the power demand changessuch as might lead to large transient increases or decreases in mixturestrength as the throttle is closed or opened.

Because of the position of the throttle in such systems, it is notpossible to use a conventional venturi carburettor metering system whichrelies on the throttle plate to play a part in metering at low load.Furthermore the alternative of a constant depression, variable geometrycarburettor metering system (typified by the well known S.U.carburettor) is complex and precluded in small engine applications onthe ground of cost.

There is an optimum mixture strength for each load, which minimisesexhaust emissions. For governed engines, or engines with a fixedrelationship between speed and load, the mixture quantity varies in aknown way with load, and can thus be used to control mixture strength.The optimum relationship is effected by engine design and must bedetermined by experiment. Current legislation in California specifiesemission of carbon monoxide, and of combined hydrocarbons and oxides ofnitrogen. In lean operation, with air/fuel ratios greater than about17:1, the nitrogen oxide emissions fall as the mixture is made leaner,while, at very lean mixtures, hydrocarbon emissions start to rise.Carbon monoxide emissions are low and practically constant for ratiosgreater than about 16:1. The result is that the total emissions are lowover a range of mixture strengths, which allows reasonable tolerance inapproximating the optimum.

Legislation specifies limits based on tests at idle, quarter load, halfload, three quarter load, and full load. For these conditions a typicalengine might require air/fuel ratios of about 17:1 at idle, 18:1 atquarter load, 19:1 at half and three quarter loads, and 12:1 at fullload. This latter rich mixture is to obtain full load while keepingtotal emissions as low as possible. The richer mixture at low load isneeded, in part, because ignition timing is fixed.

The invention seeks to provide a novel carburettor metering system whichis particularly suitable for such an application.

According to the present invention there is provided a carburettormetering system comprising evaporator means for absorbing liquid fuelfor vaporisation into an air flow to produce an air/fuel mixture, fuelmetering means for supplying fuel to the evaporator means, and airmetering means for supplying air to the evaporator means to producevaporisation of fuel supplied to the evaporator means, characterised inthat the air metering means includes an air restrictor incorporating aplurality of narrow air passages arranged adjacent to one another so asto produce substantially laminar air flow in which the pressuredifference across at least the major portion of the air restrictor issubstantially linearly related to the flow rate of air through the airrestrictor, the fuel metering means being arranged to supply fuel independence on said pressure difference.

Since it is relatively easy to meter fuel such that its flow rate issubstantially linearly proportional to the pressure difference used todrive the fuel, the use of an air restrictor producing substantiallylaminar air flow allows a substantially constant air/fuel mixturestrength to be obtained over a wide range of air flow rates regardlessof pressure fluctuations such as are encountered in single cylinderengines.

Preferably the air restrictor comprises a series of parallel platesseparated by spacers and defining narrow passages therebetween.Alternatively the air restrictor may comprise a plurality of small boretubes arranged side-by-side. In either case the pressure differenceacross the restrictor is caused primarily by viscous effects and isrelated to air flow. The relationship is substantially linear providedthat Reynolds' number, which increases with increasing flow rate andwith plate spacing or tube diameter, is less than a critical value.

In order to keep Reynolds' number below its critical value for the airflow rates likely to be encountered in use, it is necessary for therestrictor to include a large number of passages. Cost and spacerestraints have the result that the throughflow cross-section forpassage of air through the restrictor is reduced so that there is afurther pressure drop due to the fact that the cross-section of the airflow increases on being discharged from the outlet of the restrictor.This further pressure drop varies with the square of the flow rate sothat it introduces a non-linear contribution to the overall pressuredifference. In some engines the mixture strength at the maximum leanrange torque may need to be richer than that at lower loads. A smallnon-linear contribution to pressure difference can provide thisnecessary enrichment at higher flows. In other cases the existence ofthe non-linear contribution would be undesirable.

Accordingly, in accordance with a development of the invention, aventuri can be arranged upwind of the restrictor so as to provide apressure drop at the throat of the venturi which substantially offsetsthe pressure drop at the outlet of the restrictor.

As is well known, a venturi provides a pressure drop at its throatproportional to the square of the flow followed by pressure recovery sothat the outlet pressure of the venturi is substantially equal to theinlet pressure. Thus, if the pressure at the throat of the venturi isused as the reference pressure, the pressure at the outlet of therestrictor will differ from the reference pressure by a pressuredifference which is substantially linearly related to the air flow rate,provided that the pressure drop at the throat of the venturi is chosento match the pressure drop at the outlet of the restrictor, for any flowrate up to that corresponding to the critical Reynolds' number. Thecross-section and length of the restrictor can then be chosen to providea sufficient pressure difference for actuation of the required fuel flowwhilst keeping the restrictor compact and minimising the restriction toair flow.

In a preferred embodiment the fuel metering means comprises a fuelrestrictor in the form of a duct of relatively narrow cross-sectionthrough which fuel is conducted by the pressure difference across theair restrictor. For example a connection may be made between the inletof the air restrictor and a point upstream of the fuel restrictor tomaintain the fuel upstream of the fuel restrictor at the referencepressure of the inlet of the air restrictor, and a connection may bemade between the outlet of the air restrictor and a point downstream ofthe fuel restrictor to maintain the fuel downstream of the fuelrestrictor at the pressure of the outlet of the air restrictor. If themetering system includes a venturi, a connection may be made between thethroat of the venturi and a point upstream of the fuel restrictor tomaintain the fuel upstream of the fuel restrictor at the referencepressure of the venturi throat.

The invention also provides an evaporator for use in a carburettormetering system for vaporisation of fuel into an air flow passingthrough the evaporator, the evaporator comprising a series of parallellaminar elements spaced apart by spacers so as to define narrow airpassages therebetween and providing porous evaporation surfaces alongthe sides of the passages, means for supplying fuel to the elements sothat diffusion of fuel over the evaporation surfaces occurs by capillaryaction, and means for supplying air to the passages to permit fuel to beevaporated from the evaporation surfaces into the air as the air passesalong the passages.

The laminar elements may be plates of rigid porous material, such as asintered metal, or layers of fabric stretched over rigid supports withspacers therebetween.

The invention further provides a carburettor metering system comprisingevaporator means for absorbing liquid fuel for vaporisation into an airflow, fuel metering means for supplying fuel to the evaporator means,and air metering means for supplying air to the evaporator means toproduce vaporisation of fuel supplied to the evaporator means, whereinthe fuel metering means incorporates a first fuel restrictor adapted tosupply fuel from a source of fuel to the evaporator during both leanoperation and rich operation of the system, a second fuel restrictoradapted to supply additional fuel from the source of fuel to theevaporator during rich operation of the system, and switching means forchanging over from lean operation to rich operation by enabling supplyof said additional fuel to the evaporator.

The switching means preferably comprises a vent valve which is openableto vent a line connecting the second fuel restrictor to the source offuel in order to disable supply of said additional fuel to theevaporator and which is closable to enable supply of said additionalfuel to the evaporator.

The invention further provides a control device for controlling the rateof flow of air/fuel mixture to an engine, the device comprising anoutlet for supply of air/fuel mixture to the engine, a valve membermovable relative to the outlet between relatively open and closedpositions, and control means for moving the valve member relative tothe-outlet in dependence on the load of the engine, whereby, as theengine load increases, the outlet is first progressively opened by thevalve member, then at least partially closed and finally progressivelyopened again.

In order that the invention may be more fully understood, severalcarburettor metering systems in accordance with the invention will nowbe described, by way of example, with reference to the accompanyingdrawings, in which:

FIG. 1 is a block diagram of a first system;

FIG. 2 schematically shows a layout for such a system;

FIG. 3 schematically shows a section along the line A--A in FIG. 2;

FIGS. 4 and 5 respectively show a section through and a perspective viewof a valve member of the system;

FIG. 6 is an explanatory diagram;

FIGS. 7 and 8 show parts of two evaporators usable in such a system;

FIG. 9 is a schematic section through a wick and an air metering part ofa second system;

FIG. 10 is a schematic section showing the wick in greater detail; and

FIG. 11 is a diagrammatic section through a fuel metering part of thesecond system.

A first carburettor metering system in accordance with the inventionwill now be described with reference to FIG. 1 which shows a blockdiagram of the system. Air and fuel are inputted into the system by wayof inlets 100 and 101 respectively, and the required air/fuel mixture isoutputted by way of an outlet 102. The air enters the system through anair cleaner 103 and passes through a fixed pressure drop valve 104 andan air restrictor 105 to a fuel evaporator 106 before passing to theengine by way of a throttle 107. The components 104 and 105 can beinterchanged if required.

The fuel is admitted to a constant pressure fuel reservoir 108 whosepressure datum is that of the air leaving the air cleaner 103. Thereservoir 108 is typically a conventional float bowl. For lean operationfuel supplied from the reservoir 108 passes through a fuel restrictor109, which is preferably simply a narrow tube, to the evaporator 106.For rich operation additional fuel is supplied from the reservoir 108through a further fuel restrictor 110 of similar form into the air flow.The inlet to the fuel restrictor 110 is above the level at which fuel ispermitted to flow from the reservoir 108, so that such additional fuelflow can be disabled by opening a vent valve 112 having an outlet 111either to atmosphere or to the reference pressure at the outlet from theair cleaner 103. The opening of the vent valve 112 may be controlled bya cam on a governor shaft of the engine in dependence on the load of theengine.

Where the reservoir 108 is float-controlled, the level of fuel in thereservoir 108 should be below the level of the free fuel surface in theevaporator 106, in order to prevent fuel leakage when the engine is notrunning. Alternatively, where the reservoir 108 is diaphragm-controlled,the free surface of fuel in the evaporator must be higher than thepoint, set by the diaphragm offset spring, at which fuel would siphonfrom the reservoir. This imposes a need to provide a definite pressure,which is present only when the engine is running, before any fuel flows.Thus the pressure difference required to produce the necessary fuel flowmust comprise a fixed component (the offset) plus a variable componentwhich is most easily made linearly proportional to flow.

With respect to the air side of the system, an ideal arrangement wouldbe to provide an air side pressure difference having a related fixedcomponent plus a variable component proportional to the product of airflow and the required air/fuel ratio. In practice the variation inair/fuel ratio with load (and hence air flow) is met quite closely byproviding an air side pressure difference having a fixed componentsomewhat greater than the fuel side offset, with a variable componentwhich is substantially linearly related to the air flow but which has asmall positive second order term (pressure drop component proportionalto air flow squared). (If the air side fixed component were exactlyequal to the fuel side offset, and there were no second order term, theair/fuel ratio would not vary with air flow.)

The second order term provides an air/fuel ratio which increases withair flow. The amount by which the air side fixed component exceeds thefuel side offset provides a fixed additional fuel flow at all air flows,and hence a greater air/fuel ratio at low flows. Thus, by changing therelative sizes of the fixed, linear and second order terms, aprogressive variation in air/fuel ratio of any required magnitude can beproduced. The total emissions are actually found to vary only slowlywith air/fuel ratio near to the lean optimum value, so that sometolerance is available and the variation in air/fuel ratio provided bythe mechanisms proposed can be maintained within the requiredtolerances.

The system described with reference to FIG. 10 has different componentsfor the air side restrictor 105 and the evaporator 106. This isappropriate if the air resistance of the evaporator 106 variessubstantially with the amount of fuel present, such as would be the caseif the evaporator used woven fabric layers obturating the air flow. Inthis case the air passing between the strands of the fabric weave wouldbe more or less restricted as the strands swelled or shrunk with varyingfuel content. However, in another evaporator construction, the airpasses through passages formed of material such that the air passagesize is not substantially altered by varying fuel content, and in thiscase, under certain conditions, the two components 105 and 106 can becombined if required so that the evaporator itself imposes the requiredpressure difference.

A given evaporator at a given Reynolds' number has an efficiency whichis defined as the ratio of the mixture strength at its outlet to themixture strength at saturation under the given conditions of fueltemperature and air pressure. Thus an evaporator of lower efficiencymust have an outlet temperature which is higher (and hence a highersaturation mixture strength) than is ideally needed. This highertemperature reduces the charge density and hence the power of theengine. At the evaporator outlet the mixture strength immediatelyadjacent to the fuel surface is just the saturation mixture strength ofthe fuel surface. However, in the air passages remote from the fuelsurface, the mixture strength is determined by the amount of vapourwhich has diffused away from the fuel surface. Thus high efficiencyrequires that diffusion away from the fuel surface shall besubstantially complete in the time taken for the air to pass from theinlet to the outlet. In practice this requires either very small airpassages in a short structure, such as might be found in a finely wovenfabric or in several layers of more coarsely woven fabric, oralternatively a number of long passages having spaced walls wetted byfuel. Finely woven fabric has little ability to spread fuel laterally bycapillary action, and so is best suited for situations where fuel issupplied finely atomised. On the other hand relatively long passages canbe provided between plates of rigid porous material, such as a sinteredmetal or ceramic, or between layers of fabric stretched over appropriaterigid supports with suitable spacers therebetween. Alternatively thepassages can be in the form of small holes in a thick block of porousmaterial.

FIG. 2 shows a layout arrangement for such a system in which the fuelevaporator 113 is disposed in the lower part of a housing 120 so thatlower portions of plates 114 of the evaporator 113 are immersed withinfuel 121 supplied to the bottom of the housing 120 by way of a fuelinlets 122 (corresponding to the inlets from the fuel restrictors 109and 110 in FIG. 1). Air is supplied to the inlet 123 of the evaporator113 by way of a laminar flow air restrictor 128 comprising a series ofparallel plates 127 separated by spacers (not shown) and defining narrowgaps between the plates 127, and the air/fuel mixture outputted from theevaporator 113 is supplied to the engine by way of an exit tube 126.

If required, a fixed pressure drop valve (not shown) may be providedintermediate the air restrictor 128 and the evaporator 113, which iskept closed either by a weight or a light spring. In the latter case thepressure drop will not remain absolutely fixed but will vary to acertain extent with flow, thus providing a further means of adjustingthe relationship between the air/fuel ratio and the flow. Therequirement is that the valve opens when the pressure difference betweenits inlet and outlet exceeds a predetermined level, and thereafter opensprogressively with increasing flow to maintain the required pressuredrop.

The exit tube 126 comprises a slotted inlet 130 opening into the space131 within the housing 120 with which the output of the evaporator 113communicates, and is shaped so as to have a circular outlet, of the samecross-sectional area as the slotted inlet 130, which communicates withthe engine. A valve member 132 (not shown in FIG. 2) is disposedadjacent the inlet 130 and is coupled to a shaft 133 which is capable ofbeing rotated through a limited angle by the engine governor (notshown).

The valve member 132, the construction and function of which will bedescribed in more detail with reference to FIGS. 4, 5 and 6, serves tochange the system between two modes of carburettor operation, namelylean operation, which is provided up to about three quarters load, andin which fuel is supplied solely by way of the restrictor 109, and richoperation in which additional fuel is supplied by way of the restrictor110. In lean operation the throttle 107 is progressively opened by thegovernor up to full throttle as the load is increased. Thereafter, asthe load is further increased, changeover to rich operation takes place,and the throttle 107 is closed as additional fuel is introduced in orderto prevent a stepwise increase in torque. Further increase in load leadsto progressive opening of the throttle 107 again, up to full throttle.

Referring to FIGS. 5 and 6 it will be seen that the valve member 132 hasfirst and second shutters 134 and 135 separated by a slot 136, theshutters 134 and 135 and the slot 136 cooperating with the inlet 130 ofthe exit tube 126 to control the rate at which air/fuel mixture issupplied to the engine in dependence on the load on the engine. As theload is increased the valve member 132 is rotated in the direction ofthe arrow 137. Furthermore, the angular position of the valve member 132also determines whether extra fuel is added to the mixture by way of thefuel restrictor 110 by closing of the vent valve 112 (see FIG. 1).Initially, at low load, the vent valve 112 is open so that no extra fuelis added to the mixture and the air/fuel ratio varies with load as shownin the initial part of the plot 140 of this ratio against load as shownin FIG. 6. Furthermore the shutter 134 of the valve member 132 is insuch a position that it partially covers the inlet 130 to restrict theflow of mixture to the engine. As the load is increased the valve member132 is rotated in the direction of the arrow 137 so that the shutter 134uncovers the inlet 130 with the result that the open area of the inlet130 increases progressively with the load, as shown in the initial partof the plot 141 of inlet area against load as shown in FIG. 6.

When substantially the whole of the inlet 130 is uncovered by theshutter 134, which occurs at just above three quarters load, changeoverfrom lean operation to rich operation is effected by closing of the ventvalve 112 so that additional fuel is supplied to the evaporator 113 byway of the fuel restrictor 110. This transition point is indicated inFIG. 6 by the broken line 142, and it will be appreciated that theair/fuel ratio increases progressively beyond this point, as shown bythe plot 140, due to the addition of extra fuel. At the same time theshutter 135 of the valve member 132 starts to move across the inlet 130,thus reducing the open area of the inlet 130 until a maximum area of theinlet 130 is covered by the shutter 135. A further transition point isthen reached, as shown by the broken line 143 in FIG. 6, beyond whichthe inlet 130 is progressively uncovered by the shutter 135, and extrafuel is still supplied to the evaporator 113 by way of the fuelrestrictor 110. The open area of the inlet 130 then increases back to amaximum at full load, as indicated by the broken line 144 in FIG. 6.This arrangement ensures that the developed torque varies progressivelyas the valve member 132 is rotated in the direction of the arrow 137 independence on the load of the engine.

FIG. 7 shows the evaporator 113 which consists of a stack of verticalplates 114 of rigid porous material, such as sintered metal, adjacentplates 114 being separated by upper and lower spacers 115 so as todefine air passages 116 therebetween. The upper spacers 115 are flushwith the tops of the plates 114, whereas the lower spacers 115 arespaced from the bottoms of the plates 114 so as to leave short lowerportions of the plates 114 which may be immersed in the fuel. The plates114 are packed into a container so that the air is constrained to passbetween the plates 114 and along the length of the passages 116 definedby the plates 114 and the spacers 115. The fuel supplied to the lowerportions of the plates 114 migrates upwardly by capillary action so asto provide a large surface area of fuel for evaporation into the airpassing along the passages 116. In order to simplify the manufacturingprocess, each plate 114 may be formed integrally with its associatedspacers 115.

FIG. 8 shows a variant of the above described evaporator 113 in which,instead of the plates 114, a stack of solid plates 117 wrapped withfabric 118 is provided, the plates 117 being separated by spacers 119and the fabric 118 being in the form of a single sheet which is wrappedaround the spacers 119 in between wrapping of successive plates 117.

In the evaporator arrangements described above it is important that theevaporation rate of fuel should as far as possible vary linearly withthe air flow rate over a wide range of air flow rates, so that, for agiven mixture strength, the plate wetness should not change appreciablywith a change in the air flow. Variation in plate wetness can only beachieved by variation in the quantity of fuel resident in the plates ofthe evaporator, and changes in the quantity of resident fuel areundesirable because they produce transient changes in the required fuelsupply rate or transient excursions in mixture strength. Generally thearrangement should be such that Renolds' number is kept above a criticalvalue for the required range of flow rates, and various compensatingmechanisms may be applied towards this end.

For single-cylinder engines it is preferred that the evaporator is suchthat the volume of air in the evaporator is at all times comparable toor greater than the maximum volume demand per cycle of the engine. Ifrequired the effect of an intermittent air flow may be compensated forby designing the evaporator such that the air transit time within theevaporator is large with respect to the cycle time, that is so that thetotal volume represented by the product of the plate area and the gapbetween the plates is large compared with the maximum ingested volume ofair per cycle, so as to give the longest possible time for fuel vapourto diffuse into the air stream.

For multi-cylinder engines, on the other hand, the air flow through theevaporator is more nearly continuous. In this case it is necessary toensure that the evaporator is designed such that the Renolds' number isabove the critical value for the average minimum air flow through theevaporator. However, for a fixed geometry, the pressure drop imposed bythe evaporator varies in proportion to the square of the air flow, withthe result that this pressure drop would become very large at themaximum air flow if the design of the apparatus is optimised for thelowest air flow through the evaporator. In order to avoid such a largepressure drop at maximum flow, various compensating mechanisms may beapplied. For example an adjustable compensating plate may be providedwhich serves to progressively uncover the gaps between the evaporatorplates, in either a linear or a stepwise manner, so that an increasingnumber of passages become available for the air flow through theevaporator as the air flow rate increases, thus maintaining Renolds'number within the required range. If such a compensating mechanism isprovided, it is not essential that the volume of air in the evaporatoris maintained comparable to or greater than the maximum volume demandper cycle of the engine.

Referring to FIG. 9, a second carburettor metering system in accordancewith the invention includes, within a common housing 2, a wick 4 and alaminar flow air restrictor 8. The air restrictor 8 divides the housing2 into a wick chamber 14 and a lower chamber 13. The housing 2 isprovided with an exit tube 7 leading to the engine via the throttle (notshown). The exit tube 7 is provided with a half round baffle 9 forpreventing droplets of fuel from entering the engine. The wick 4comprises a wire metal support grid 10 having two inclined flat portions11 and 12 arranged to form a V-shaped cross-section, and extendingbetween opposite side walls of the wick chamber 14.

The construction of the wick is shown in greater detail in FIG. 10. Theupwind and downwind sides 15 and 16 of the support grid 10 are eachcovered by a layer 17 or 18 of relatively tightly woven fabric which isof sufficiently fine mesh to prevent droplets of liquid fuel passingtherethrough. This tightly woven fabric is particularly efficient atevaporating the fuel. Furthermore a layer 20 of relatively loosely wovenfabric is applied to the upwind face only of the support grid 10,between the support grid 10 and the layer 18 of tightly woven material,to absorb liquid fuel supplied to the wick from a spreader tube 22 (seeFIG. 9) and to permit lateral spread of the fuel by capillary actionover the whole of the layer 20 so as to maximise the surface area offuel available for evaporation into the air flow passing through thewick 4.

The spreader tube 22, which extends along the top of the wick 4 and isembedded within the layer 20 of loosely woven fabric, is formed withsmall holes along its length, through which fuel is supplied to the wick4. In order to provide accurate control of the air/fuel mixture strengthand to avoid fluctuations in mixture strength over the engine cycle,fuel is supplied to the wick 4 by the spreader tube 22 in dependence onthe pressure difference between first and second pressure openings 19and 21 located respectively near the inlet 23 of the air restrictor 8and near the outlet 25 of the air restrictor 8. Since the air restrictor8 comprises a series of parallel plates 24 separated by spacers (notshown) and defining narrow gaps between the plates 24, the air flowthrough the air restrictor 8 produces a pressure difference between theends of the air restrictor 8 that is substantially linearly related tothe air flow rate through the restrictor 8 (provided that the airrestrictor 8 is sufficiently large).

Referring to FIG. 11 the fuel metering part 28 of the system, which ishoused within the lower chamber 3, comprises a fuel reservoir 30containing fuel 32 up to a level 33 determined by a float 34 within thereservoir 30. The reservoir 30 has a fuel inlet 36 and is vented by apressure tube 37 connected to the first pressure opening 19 so as toapply the pressure at the first pressure opening 19 to the fuel 32 inthe reservoir 30. The reservoir 30 is connected by a duct 38 to a fuelrestrictor 40 comprising a length of small bore tubing 41, and the fueloutputted from the fuel restrictor 40 emerges into a well 50.

The well 50 is in turn connected to a further well 54 so that fuel issupplied to the well 54 by spilling over a weir 42 from the well 50. Theweir 42 defines the height of the fuel at the outlet from the restrictor40 relative to the float level 33. The wells 50 and 54 are both ventedby a pressure tube 56 which is connected to the second pressure opening21 so as to apply the pressure at the second pressure opening 21 to thefuel in the well 50 thus providing a pressure difference between thepressure tubes 37 and 56 which is linearly proportional to the air flowrate for controlling fuel flow through the fuel restrictor 40. Fuel 32is supplied from the well 54 to the spreader tube 22 in the wick chamber14 by way of a fuel supply duct 57. In order to apply the necessarypressure drop to permit fuel to be conducted along the duct 57, the airflow is conducted from the outlet 25 of the air restrictor 8 through ahinged weighted flap 58 (see FIG. 9) prior to being introduced into thewick chamber 14. The well 50 provides a free surface 59 of fuel beneaththe level 33 of fuel 32 in the reservoir 30 and thus prevents anyirregularities of fuel supply to the spreader tube 22 due to surfacetension effects.

The described fuel restrictor 40 provides accurate control of fuelmetering since the fuel flow rate through the fuel restrictor 40 islinearly related to the pressure difference between the first and secondpressure openings 19 and 21 which is itself linearly related to the airflow rate. However, in addition to the pressure difference between thetwo ends of the air restrictor 8, there is a further pressure drop,corresponding to the difference between the pressure at the outlet 25 ofthe air restrictor 8 and the pressure at the pressure opening 21, andmagnitude of this pressure drop varies with the square of the flow rate.Strictly this pressure drop occurs on entry into the air restrictor 8due to the decrease in throughflow cross-section caused by the presenceof the plates 24, although the pressure drop only becomes apparent ondischarge of the air from the spaces between the plates 24 at the outlet25 as the pressure drop is not recovered at the outlet 25. This squarelaw pressure drop becomes more significant as the air restrictor 8 ismade smaller, such as might be the case, for example, in a chainsawmotor.

If it is necessary to compensate for this square law pressure drop atthe outlet 25 of the restrictor 8, the heated air flow supplied to therestrictor 8 may, before entering the restrictor 8, be first passedthrough a venturi (not shown) which produces a pressure drop at thethroat of the venturi whose magnitude again varies with the square ofthe flow rate. Downwind of the venturi throat the pressure recovers sothat the pressure at the inlet 23 of the restrictor 8 is substantiallythe same as the pressure at the inlet of the venturi. The pressure tube37 may then be connected to the venturi throat so that the pressure atthe venturi throat is taken as the reference pressure and the square lawpressure drop at the venturi throat is arranged to compensate for thesquare law pressure drop at the outlet 25 of the air restrictor 8. Itwill be appreciated that the pressure difference between the pressuretubes 37 and 56 will then be substantially linearly related to the airflow rate for flow rates having a Renolds' number which is less than acritical value.

The exit tube 7 is preferable provided with a valve member (not shown)for changing the system from lean operation to rich operation in asimilar manner to that already described with reference to the previousembodiment. As shown in FIG. 11 a flange 70 which extends around theexit tube 7 is provided with a vent aperture 84 and a fuel enrichmentaperture 86. The vent aperture 84 is connected via a vent conduit 88 tothe pressure tube 37, and the fuel enrichment aperture 86 is connectedvia a fuel conduit 90 to the duct 38. The angular position of the valvemember, which is controlled in dependence on the engine load, determinesthe rate at which mixture is supplied to the engine and also determineswhether extra fuel is added to the mixture by way of a fuel conduit 90incorporating a fuel restrictor 92 connected to the fuel enrichmentaperture 86, in a similar manner to that already described withreference to the previous embodiment.

Such a carburettor metering system is particularly advantageous for usewith single cylinder four-stroke engines of low cost, such as are usedin lawnmowers for example, and permits accurate control of air/fuelmixture strength in spite of the intermittent nature of supply ofmixture to such an engine. However, the system may also be used withmulti-cylinder engines. The use of a throttle downstream of thecarburettor metering system minimises mixture strength excursions duringtransients, as are obtained, for example, on increase or decrease ofengine load.

I claim:
 1. A carburettor metering system comprising evaporator meansfor absorbing liquid fuel for vaporisation into an air flow to producean air/fuel mixture, fuel metering means for supplying fuel to theevaporator means, and air metering means for supplying air to theevaporator means, characterised in that the air metering means includesan air restrictor incorporating a plurality of narrow air passagesarranged adjacent to one another so as to produce substantially laminarair flow in which the pressure difference across at least the majorportion of the air restrictor is substantially linearly related to theflow rate of air through the air restrictor, the fuel metering meansbeing arranged to supply fuel in dependence on said pressure difference.2. A system according to claim 1, wherein the air passages of the airrestrictor are straight passages arranged parallel to one another.
 3. Asystem according to claim 2, wherein the air restrictor comprises aseries of parallel plates separated by spacers and defining said airpassages therebetween.
 4. A system according to claim 1, wherein aportion of the air restrictor introduces a small pressure drop whichvaries with the flow rate of air through the air restrictor so as toprovide an air/fuel mixture of increased strength at high air flowrates.
 5. A system according to claim 1, wherein valve means areprovided in series with the air restrictor for applying a substantiallyfixed pressure drop to the air flow.
 6. A system according to claim 1,wherein the fuel metering means comprises a fuel restrictor throughwhich fuel is conducted by the pressure difference across the airrestrictor.
 7. A system according to claim 1, wherein enrichment meansare provided for adding additional fuel to the air/fuel mixture when theload exceeds a predetermined level.
 8. An evaporator for use in acarburettor metering system for vaporisation of fuel into an air flowpassing through the evaporator, characterised in that the evaporatorcomprises a series of parallel laminar elements spaced apart by spacersso as to define narrow air passages therebetween and providing porousevaporation surface along the sides of the passages, means for supplyingfuel to the elements so that diffusion of fuel over the evaporationsurfaces occurs by capillary action, and means for supplying air to theair passages to permit fuel to be evaporated from the evaporationsurfaces into the air passing along the air passages.
 9. A carburettormetering system comprising evaporator means for absorbing liquid fuelfor vaporisation into an air flow, fuel metering means for supplyingfuel to the evaporator means, and air metering means for supplying airto the evaporator means to produce vaporisation of fuel supplied to theevaporator means, wherein the fuel metering means incorporates a firstfuel restrictor adapted to supply fuel from a source of fuel to theevaporator means during both lean operation and rich operation of thesystem, a second fuel restrictor adapted to supply additional fuel fromthe source of fuel to the evaporator means during rich operation of thesystem, and switching means for changing over from lean operation torich operation by enabling supply of said additional fuel to theevaporator.